Abnormal low-temperature behavior of a continuous photocurrent in Bi2S3 nanowires

High-quality Bi2S3 nanowires are synthesized and their photoresponses are investigated in detail. Our results show that the photoresponsive curves have distinctly different characteristics at low-temperature (50 K) compared to those at room temperature (290 K). The transferred-electron effect is believed to cause this difference. A first principle calculation shows that Bi2S3 has many energy valleys, which agree with our experimental analysis. At low temperature, due to the lack of sufficient phonon energy, the photoexcited electrons in Bi2S3 mainly aggregate at the bottom of the conduction band. When this electron concentration increased to a high enough level after illumination, an electron transfer between the energy valleys happened and the photocurrent began to decrease slowly after the rapid increase in the first stage. After the transfer process reaches equilibrium, the photocurrent reaches a minimum, thus the trap states play a dominant role and the photocurrent rises slowly again. Furthermore, photocurrent curves at different temperatures were recorded to estimate the phonon energy value needed to assist the electron transitions. The required phonon energy is calculated to be about 16.3 meV (corresponding to 190 K), which fits well with previous results.